During blasting of caverns, one will as a general rule break the groundwater-leading channels and crevices in the rock so that more or less permanent water leakages occur into the cavern. Consequently there is a need to lead this water away, and this has been previously been solved in different ways. Thus, different funnel and duct constructions with associated pipes that lead the water to a drainage ditch or to a similar drainage system at the floor of the cavern have been used. For example, reference is made to Norwegian patent application 1999 2485 belonging to the present applicant that shows different ditch solutions. It is also common to build in constructions of wood, steel or concrete, encompassing a covering of walls and ceiling with different types of ceiling boards and wall covering boards. Examples of this are shown in the German patents 874.317 and 926.255. It is also common to fasten such ceiling boards and wall boards onto the exposed sections of the walls and ceiling only, where there is a danger for ingress of water. Spray concrete is also used to a large extent to cover such areas. Furthermore, it is common to use different types of vault constructions to seal off the vault ceiling, and to use concrete directly against the rock, possibly with a sealing membrane.
It is also previously known to put up a watertight canvas vault corresponding to the profile of the cavern, so that a space is formed between the canvas and the rock, and where the canvas, along its two lower edge sections (along the ditch sections), runs over into the ground material such as gravel, sand and the like to form a seal. The space between the canvas and the rock wall constitutes the wet room, while the workspace underneath the canvas vault, inside the canvas surface, is the dry room.
One of the abovementioned canvas solutions, which in reality is only used sporadically these days, involves the lower edge sections, or flaps of the canvas pieces, being arranged with a U-shaped course which forms a canal that is formed in the floor of the cavern along the lower edge of the walls. The hollow space of the U-shape can be filled with gravel, while underneath (and partially beside) the U-shape, there can be coarser gravel. A concrete covering, an asphalt covering or similar floor covering, can be arranged close up to the hollow space of the U-shape.
Reference is also given to what is known from Swedish patent 457.549 and U.S. Pat. No. 6,308,479.